Well, you can install cygwin
http://www.cygwin.com and use bash under windows
You could also try 4NT from
http://www.jpsoft.com.
Otherwise, I don't know of a way to do this other than writing batch files (but you couldn't create replacements for builtin commands like dir - that is, if you made a dir.bat, it wouldn't be run by default if you typed dir, regardless of your PATH).
On the other hand, if you want to alter the default dir, you can set the DIRCMD environment variable (see dir /?), but this is specific to each command.